The jet stream is a narrow band of strong winds high in Earth’s atmosphere that helps steer weather systems around the planet. These winds influence where storms travel, how quickly weather changes, and whether regions experience extended periods of warmth, cold, rain, or dry conditions. Although the jet stream flows thousands of feet above the ground, its position and movement can have a significant effect on everyday weather across the United States.
Jet streams are not solid rivers of air with clearly defined boundaries. They are elongated regions of fast-moving air embedded in the atmosphere, typically found near the boundary between the troposphere and the stratosphere. The troposphere is the lowest layer of the atmosphere, where nearly all weather occurs.
Understanding how the jet stream forms and changes helps explain why weather patterns can shift rapidly in some situations but remain stubbornly similar for days or even weeks in others.
What is the jet stream?
The jet stream consists of powerful, high-altitude air currents that generally flow from west to east in the middle and upper latitudes. The strongest winds are concentrated in relatively narrow bands, although their width, altitude, and wind speed vary.
The main jet streams commonly discussed in weather forecasts are the polar jet stream and the subtropical jet stream. Both form in response to large-scale differences in temperature and atmospheric circulation, but they occur in different latitude ranges and have somewhat different influences on weather.
The polar jet stream is particularly important for weather in the United States. It often separates colder air masses to the north from warmer air masses to the south. Its position helps determine where cold fronts, warm fronts, and many low-pressure systems develop and move.
The subtropical jet stream typically forms farther south, near the boundary between tropical and subtropical atmospheric circulation. It can influence storm tracks, particularly during certain seasons and in parts of the southern United States.
Neither jet stream follows a fixed path. Both can shift north or south, strengthen or weaken, and develop broad curves that change the distribution of weather across large areas.
Jet streams generally occur at altitudes of roughly 30,000 to 40,000 feet, or about 9 to 12 kilometers, although their height varies with location and atmospheric conditions. Winds within their strongest cores can exceed 100 miles per hour, and substantially higher speeds sometimes occur.
These winds are important not because they directly create every weather event below them, but because they help organize and steer the atmospheric systems responsible for much of the weather people experience.
How does the jet stream form?
The jet stream develops because Earth receives uneven amounts of solar energy and because the planet rotates.
The equator receives more direct sunlight throughout the year than the polar regions. As a result, tropical regions are generally warmer than high latitudes. This temperature contrast affects atmospheric pressure and creates large-scale differences in how air moves through the atmosphere.
Warm air expands, while cold air is denser. These differences contribute to variations in atmospheric pressure with altitude. Where large horizontal temperature differences exist, the pressure pattern changes with height in a way that helps generate strong winds aloft.
Earth’s rotation also plays a crucial role. The Coriolis effect is the apparent deflection of moving air caused by Earth’s rotation. In the Northern Hemisphere, moving air is deflected to the right relative to its direction of travel; in the Southern Hemisphere, it is deflected to the left.
Together, the temperature contrasts and the Coriolis effect help produce strong winds that tend to flow along the boundaries between warmer and colder air.
A useful principle in meteorology is that stronger horizontal temperature contrasts are associated with stronger vertical changes in wind speed. This relationship helps explain why the polar jet stream can intensify where cold polar air meets much warmer air farther south.
The jet stream is also shaped by the broader circulation of the atmosphere. Air rises in some regions, sinks in others, and moves between the tropics and higher latitudes. These circulation patterns interact with continents, oceans, mountain ranges, and developing weather systems.
The result is a dynamic system rather than a uniform band of wind. The jet stream continually responds to changing temperature patterns and atmospheric pressure, making its behavior an important part of weather forecasting.
Why does the jet stream flow from west to east?
Although winds can blow in many directions, the major jet streams in the middle latitudes generally move from west to east. This direction results largely from the combination of Earth’s rotation and the large-scale temperature and pressure patterns in the atmosphere.
At high altitudes, air responds to horizontal pressure differences while being deflected by the Coriolis effect. The resulting flow often runs approximately parallel to lines of equal pressure, rather than directly from high pressure toward low pressure. This is known as geostrophic flow when the pressure-gradient force and Coriolis effect are approximately balanced.
In the middle latitudes, these processes favor strong west-to-east winds. The temperature contrasts that help form the jet stream also influence how quickly wind speed changes with altitude, reinforcing the strong upper-level flow.
The jet stream does not always move straight east, however. It frequently bends north and south in large waves. These waves, known as Rossby waves, arise from the effects of Earth’s rotation on large-scale atmospheric motion and are influenced by the distribution of land, oceans, mountains, and temperature.
When the jet stream curves northward, warmer air can extend toward higher latitudes. When it curves southward, colder air can move toward lower latitudes. These bends help explain many of the major shifts in weather across North America.
How the jet stream steers storms
One of the jet stream’s most important functions is guiding the movement and development of weather systems.
Many large low-pressure systems in the middle latitudes form and move in association with disturbances in the jet stream. These systems often bring clouds, precipitation, and changes in temperature. The jet stream helps establish their preferred paths, commonly called storm tracks.
Its influence is more complex than simply pushing storms along from behind. Changes in wind speed and direction, especially near the jet stream’s strongest regions, can promote rising or sinking air through changes in the atmospheric pressure field.
For example, certain upper-level wind patterns encourage air to spread out at high altitude. When air is removed from a column of the atmosphere, air from below may rise to replace it. Rising air expands and cools, allowing water vapor to condense into cloud droplets or ice crystals when conditions are suitable.
If enough moisture is present, this process can contribute to rain or snow. When rising motion occurs over a broad area and persists, it can help a low-pressure system intensify.
Other upper-level patterns favor sinking air. Sinking air compresses and warms, which often reduces relative humidity and discourages cloud formation. This can support clearer, drier weather.
The relationship is not automatic. Moisture availability, surface temperatures, fronts, terrain, and the structure of the weather system all influence the outcome. A strong jet stream does not guarantee a storm, and a weak jet stream does not rule one out.
Nevertheless, the position and shape of the jet stream provide forecasters with valuable clues about where storms may form, how they may strengthen, and which areas are likely to receive precipitation.
How the jet stream affects temperature
The jet stream can influence temperature by helping determine the paths taken by warm and cold air masses.
An air mass is a large body of air with relatively consistent temperature and moisture characteristics. In North America, cold air from Canada and the Arctic can move southward, while warmer air from the Gulf of Mexico and lower latitudes can travel northward.
The jet stream often marks a transition zone between these contrasting air masses. When its main path shifts south, colder air can gain access to regions farther south. When it shifts north, warmer air can spread into areas that might otherwise experience cooler conditions.
These patterns help explain why a city can experience an abrupt temperature drop after a cold front passes or several unusually warm days when southerly flow persists.
The jet stream does not determine surface temperatures on its own. Clouds, sunlight, snow cover, soil moisture, ocean temperatures, and surface winds also matter. A southward dip in the jet stream creates conditions that can favor cold weather, but the actual temperature depends on the air that arrives and how it interacts with the local environment.
Likewise, a northward bend does not guarantee a heat wave. Persistent warmth typically requires a combination of suitable air masses, atmospheric circulation, and local conditions.
What are jet stream dips and ridges?
The jet stream often takes on a wavy pattern rather than flowing in a straight line. Two important features of this pattern are troughs and ridges.
A trough is an elongated region of relatively low atmospheric pressure or lower geopotential heights aloft. On weather maps, it commonly appears as a southward dip in the flow. A trough in the jet stream can allow colder air to move south and may support rising air, cloud formation, and storm development.
A ridge is an elongated region of relatively high atmospheric pressure or higher geopotential heights aloft. It often appears as a northward bulge in the flow. Ridges are frequently associated with warmer air moving poleward and with sinking air that favors clearer or drier conditions.
These features help explain why weather can differ sharply between regions at the same time. A trough may bring chilly, unsettled weather to one part of the country while a ridge supports mild or warm conditions elsewhere.
Ridges and troughs also interact with one another. Their movement, strength, and position influence how air flows through the atmosphere and where weather systems are most likely to develop.
The waves can move eastward, change shape, deepen, or weaken. Sometimes they progress steadily across the country, bringing a sequence of changing weather conditions. At other times, they move slowly or become unusually persistent.
Why the jet stream sometimes stalls
The jet stream does not always carry weather systems rapidly from west to east. Its waves can slow down, allowing similar atmospheric conditions to persist over a region.
When a ridge remains in place, sinking air and persistent warm conditions can contribute to an extended period of heat or dryness. If a trough remains nearby, cool weather and repeated rounds of precipitation may continue for several days.
A slow-moving weather pattern can increase the risk of extreme conditions because the same region remains exposed to them. Persistent heat can worsen drought conditions and increase stress on people, crops, and ecosystems. Repeated storms over the same area can increase the risk of flooding, especially when soils are already saturated.
One feature sometimes associated with slow-moving patterns is a blocking high, a persistent area of high pressure that disrupts the usual eastward progression of weather systems. Blocking can cause the jet stream to divert around the high-pressure region or remain displaced from its usual path.
However, not every prolonged heat wave, cold spell, drought, or flood is caused by a stalled jet stream. Surface conditions and other atmospheric processes also contribute, and the details differ from one event to another.
A persistent jet stream pattern can make extended weather more likely, but it does not determine exactly how long an event will last or how severe it will become.
How the jet stream changes with the seasons
The jet stream shifts throughout the year as the temperature contrast between the equator and the poles changes.
During winter in the Northern Hemisphere, polar regions become much colder while tropical regions remain comparatively warm. This often strengthens the temperature contrast across the middle latitudes, helping the polar jet stream become stronger and more variable.
The jet stream also tends to extend farther south during winter, although its actual position changes from day to day. This can allow cold air outbreaks to reach the central and southern United States while warmer air moves northward in other areas.
During summer, the polar regions warm relative to winter, generally reducing the temperature contrast that drives the polar jet stream. The polar jet often shifts northward and weakens, though strong winds and significant waves can still occur.
The subtropical jet stream has its own seasonal behavior. It is often stronger during the cool season in some regions and can influence the distribution of storms and precipitation farther south.
These are broad tendencies rather than strict rules. The jet stream does not follow the same schedule every year, and temporary weather patterns can depart considerably from seasonal averages.
Its seasonal movement is one reason the United States experiences different storm tracks, temperature patterns, and precipitation regimes throughout the year.
The jet stream and severe weather in the United States
The jet stream can help create an environment favorable for severe thunderstorms, including storms capable of producing damaging winds, large hail, and tornadoes.
Severe thunderstorms often develop when warm, moist air near the surface interacts with cooler or drier air above it, alongside a mechanism that lifts the surface air. The jet stream can contribute to this setup by influencing upper-level winds, atmospheric instability, and the organization of developing storms.
One important factor is wind shear, the change in wind speed or direction with height. Strong vertical wind shear can help thunderstorms become organized and, under suitable conditions, sustain rotating updrafts. An updraft is a rising current of air within a storm.
The jet stream can contribute to strong winds aloft and therefore to wind shear, particularly when its position aligns with favorable surface conditions. In combination with sufficient moisture, instability, and lifting, this can support the development of organized severe thunderstorms.
However, the jet stream alone does not cause tornadoes or guarantee severe weather. Many other atmospheric ingredients must come together, and some thunderstorms develop in environments where the jet stream plays a less direct role.
The jet stream also influences winter storms. Its position can help determine where cold air meets moist air, affecting whether precipitation falls as rain, snow, sleet, or freezing rain. Small changes in the storm track and temperature profile can produce substantial differences in precipitation type across nearby locations.
Understanding the jet stream is therefore important for forecasting both ordinary changes in weather and potentially dangerous events.
Does the jet stream affect hurricanes?
The jet stream can influence hurricanes and other tropical cyclones, but its effects differ from those on typical midlatitude storms.
Hurricanes usually form over warm tropical or subtropical oceans and are powered by heat and moisture from the ocean. They are not primarily driven by the temperature contrasts that generate the polar jet stream.
Nevertheless, upper-level winds and larger atmospheric circulation patterns can affect a hurricane’s movement, strength, and eventual transition into a different type of storm.
Winds in the surrounding atmosphere help steer a hurricane. The jet stream may influence this steering indirectly by shaping the broader wind pattern, especially when a hurricane moves toward higher latitudes.
Strong winds aloft can also increase vertical wind shear. If the winds at different heights become too different in speed or direction, they can tilt a hurricane’s circulation and disrupt the alignment of its thunderstorms. This often weakens a tropical cyclone, although the outcome depends on the storm’s structure and the surrounding environment.
As a hurricane moves over cooler water or interacts with a midlatitude weather system, it may undergo extratropical transition. During this process, the storm loses its primarily tropical structure and begins to derive more of its energy from horizontal temperature contrasts. Interaction with the jet stream can help facilitate this transition and sometimes contribute to the development of a powerful post-tropical storm.
The relationship is complex: the jet stream can influence a hurricane without directly controlling it, and its effects depend on the storm’s location, intensity, and surrounding atmospheric conditions.
How scientists track and forecast the jet stream
Meteorologists monitor the jet stream using weather balloons, aircraft observations, satellites, radar data, and other measurements that reveal atmospheric conditions at different altitudes.
Weather balloons carry instruments that measure temperature, pressure, humidity, and wind as they rise through the atmosphere. Aircraft provide additional wind and temperature observations, while satellites help scientists estimate atmospheric motion and observe large-scale patterns over regions where direct measurements are limited.
These observations are combined with numerical weather prediction models. Such models use mathematical descriptions of atmospheric physics to estimate how winds, temperatures, pressure, and moisture will change over time.
Upper-level weather maps often display the jet stream using wind-speed shading, wind symbols, or lines representing geopotential height. Geopotential height describes the altitude of a particular atmospheric pressure level and helps meteorologists identify ridges, troughs, and the broader flow pattern.
Forecasters examine the jet stream alongside surface pressure systems, fronts, moisture fields, and temperature patterns. This combined view is more useful than considering the jet stream in isolation.
Forecast uncertainty generally increases farther into the future because small differences in the estimated starting conditions can lead to increasingly different atmospheric outcomes. A forecast may capture the broad movement of the jet stream while remaining uncertain about the exact timing, strength, or location of an individual storm.
Even with these limitations, monitoring the jet stream is essential for predicting major weather changes across the United States.
How climate change may affect the jet stream
Climate change is warming the atmosphere unevenly, with the Arctic warming faster than the global average in recent decades. This has raised important questions about how the temperature contrast between the Arctic and lower latitudes may affect the polar jet stream.
Because the jet stream is influenced by horizontal temperature differences, changes in those differences can alter atmospheric circulation. However, the relationship is complicated by other influences, including tropical temperature patterns, ocean variability, changes in atmospheric moisture, and natural fluctuations in the climate system.
Scientists continue to investigate whether and how Arctic warming affects the jet stream’s typical waviness, persistence, and tendency to produce prolonged weather patterns. Research has not established a simple, universal relationship in which Arctic warming necessarily causes a wavier or more frequently stalled jet stream.
Climate change can affect extreme weather through several mechanisms that are better established. Rising global temperatures increase the likelihood and intensity of extreme heat in many regions, while a warmer atmosphere can hold more water vapor, increasing the potential for heavy precipitation when weather conditions support it.
The jet stream remains an important part of the atmospheric circulation involved in these events, but it is only one part of a much larger system. Changes in future weather patterns will depend on interactions among global warming, natural variability, ocean conditions, and atmospheric dynamics.
Why the jet stream matters for everyday weather
The jet stream is an important link between large-scale atmospheric circulation and the weather people experience on the ground. By steering storms, separating contrasting air masses, and influencing rising and sinking air, it helps shape temperature changes, rainfall, snowfall, and periods of persistent weather.
A shift in its position can change which regions receive cold air, where storms travel, and whether an area experiences unsettled weather or extended warmth and dryness. Its influence is especially noticeable when broad waves become unusually strong or remain in place for an extended period.
Yet the jet stream is not a simple weather switch. It interacts continuously with the rest of the atmosphere, and its effects depend on moisture, surface conditions, pressure systems, and many other factors.
The most useful way to understand the jet stream is as a moving part of Earth’s atmospheric circulation: a high-altitude current that helps organize weather across continents and oceans, linking conditions far above the ground to the changing skies below.
